1. Field of the Invention
This invention relates generally to vapor deposition apparatus and methods, and specifically to a fixture for vapor phase diffusion coating of airfoil components.
2. Discussion of the Prior Art
Turbine components such as airfoils require coatings such as aluminides for environmental protection or as bond coats. However, only a portion of the airfoil that is exposed to the hot gases of the environment requires such protection, the remainder of the airfoil not requiring such protection. Not only does the remainder of the airfoil not require such protection, coating of that portion of the airfoil is undesirable.
Turbine airfoils have an airfoil section which extend outward into the hot gases resulting from the combustion of fuel in the combustor portion of the engine. These hot gases provide the energy for operation of the engine and for thrust to propel a turbine powered vehicle. Because of the high temperatures and corrosive effects of such gases on the airfoil section, the standard practice has been to coat the airfoil portions of the turbine airfoils with protective coatings that provide insulation from the extremely high temperatures and environmental protection from the corrosive effects of the hot gases. The portion of the turbine airfoil opposite the airfoil portion is used to attach the airfoil to the disk or rotor part of the engine, which is not in the flow of hot gases and therefore is not in need of protection from heat or corrosive effects. This portion of the turbine airfoil often has the shape of a dovetail, which is assembled into dovetail slots on the disk or rotor portion of the engine. Hence, it is frequently referred to as the dovetail. The walls of the dovetail portion of the turbine airfoil contact the walls of the dovetail slots of the disk or rotor. After a long period of time or rotating at high speeds, the dovetail walls exhibit a fatigue-related phenomenon referred to as fretting. Fretting has been found to be exacerbated by coatings applied to the airfoil portion of the turbine airfoil. Thus, in order to achieve the desired properties in the various portions of the turbine airfoil to maximize the life of the turbine airfoil, it has been necessary to devise methods to properly coat the airfoil portion of the turbine airfoil without affecting the dovetail portion of the turbine airfoil.
One of the methods of providing the coating to the desired portion of the turbine airfoil has been to mask the portion of the turbine airfoil that does not require coating, that is to say, the dovetail, before inserting the turbine airfoil into a coating apparatus. Typically, two different types of masks are applied. One type of mask is a material that is inert to the metallic gases used in the coating process. A second type of mask is a material that is a getter for the metallic gases used in the coating process, that is to say, the gases preferentially are deposited on the getter material than on adjacent exposed portions of the dovetail. Both types of masks can be applied as a coating, slurry or as a tape. The difficulty with coatings and slurries is that they can enter into cooling holes. They are difficult to remove from such passages and can block or restrict the flow of air through cooling passages causing hot spots on the turbine airfoil. They are also time-consuming to apply and are difficult to remove. The problem with tapes is that they may not be sufficiently adherent during the coating process, so that coating material can reach the turbine airfoil through seams that may open up during the coating process. Additional problems exist with gettering materials used as masks. They can become saturated and, hence, ineffective after a period of time. Additionally, because they getter the metallic vapor within the coating apparatus, additional time at a given vapor pressure or additional vapor pressure at a fixed time is required to coat the portions of the turbine airfoil that are desirably coated. Finally, if the gettering materials are not properly mixed and applied, they can react with the dovetail portion of the blades, causing the very problem they were intended to prevent.
Fixtures are used in certain applications to hold substrates as they are being coated. One such device is set forth in U.S. Pat. No. 4,485,789 to Brandolf. This patent describes an apparatus for holding a substrate as the substrate is being rotated. The substrate is rotated to provide uniform coating, as the coating method involves line of site application. Planetary motion of substrate improves coating uniformity. The device utilizes graphite bearing means between the substrate mounting head and the base support of the device to act as a sliding bearing surface between the head and the base member to prevent sticking and binding of the rotating parts, and to provide an electrical conductive path between the base support and the substrate mounting head.
What is needed is improved apparatus and method for coating turbine airfoils. The apparatus and method should permit coating application to only those portions of the airfoil requiring protective metallic coatings while protecting those portions not requiring a protective coating. Furthermore, to be effective, the improved apparatus and method must be reusable and capable of reducing the current cycle time to prepare a turbine airfoil for coating, while providing a coating at least of similar quality to coatings currently applied.